Many outdoor surfacing applications have employed asphalt for its weather resistance and superior mechanical properties under heavy loads. Asphalt is a dark cementitious material composed predominantly of bitumens. Most asphalts are now produced from the refining of petroleum and are used primarily in paving and roofing applications. At normal service temperatures asphalt is viscoelastic; at higher temperatures, it becomes viscous.
The water resistance of asphalt layers is essential to its durability. Asphalts that have a low content of soluble salts show a low water absorption. When asphalt picks up water, it softens and blisters. Bacteria and fungi are also known to attack the very low molecular weight portion of bituminous materials. Exposed asphalt films have, additionally, been known to harden and crack when exposed to ultraviolet radiation.
Asphalt-mastics are known to include mineral fillers which are added to influence their flow properties and reduce costs. Mineral-filled films show proven resistance to flow at elevated temperatures, improved impact resistance and better flame-spread resistance. Fillers may also increase the water absorption of asphalt, and can include ground limestone, slate flowers, finely divided silica, trapped rocks, and mica. Opaque fillers offer protection from weathering, and asbestos filler, because of its fibrous structure can be added to improve toughness. Asbestos fibers have also been added to asphalt paving mixes to increase the resistance to movement under traffic, and in roofing materials, for fire-retardant purposes.
Numerous asphalt-based membranes have been produced for smaller surface-area applications, such as in the patching of roads and roofs. Such membranes can be non-reinforced, or lightly reinforced and are known to have pressure sensitive characteristics. One example of a commercial membrane product, Road Glass from Owens-Corning Fiberglass, provides a membrane capable of accepting an embedded reinforcement at the job site. The Road Glass system involves melting asphalt-based mastic onto the targeted surface, followed by the application of a fiberglass layer, and then, an additional layer of asphalt mastic is provided to produce a composite membrane.